oven, to eliminate the ammonium nitrate, the temperature being raised
gradually to 160-180°; excessive heating must, however, be avoided as
it entails the danger of reducing the molybdic acid. If a dry, cold watch-
glass placed on the crucible does not become dimmed in about half a minute,
the expulsion of the ammonium salts is complete and the hot crucible is
allowed to cool in a desiccator and weighed as rapidly as possible to prevent
absorption of moisture. (Ammonium phosphomolybdate) X 0-0164 —
As already mentioned, phosphorus may be determined directly in the liquid
from which the silica has been filtered, in the case when the metal was dissolved
in nitric acid. In this case the hydrochloric acid solution freed from silica is
evaporated to a syrup in a 250 c.c. beaker, allowed to cool (no basic iron salt
should separate) and 50-100 c.c. of molybdate reagent added according to the
quantity of phosphorus present, the remaining procedure being as above.
When a large amount of substance has been taken for the silicon estimation,
the phosphorus determination is made on an aliquot part of the nitrate.
Further, if a very large amount of phosphomolybdate separates, some prefer
to dissolve it in ammonia and filter, and precipitate by means of magnesia mix-
ture. In this case, the magnesium ammonium phosphate is collected on a filter
after 6-7 hours, and washed with slightly ammoniacal water until the washings
pass through free from chlorine, the paper precipitate being incinerated
together at not too high a temperature and then strongly ignited to constant
weight in a blowpipe flame : Mg2P2O7 x 0-2787 = P.
(6) DIRECT WEIGHING OF THE AMMONIUM PHOSPHOMOLYBDATE (Rapid
method).—5 grams of iron or steel of low phosphorus content, or i--2
grams of cast-iron, or 0-5-1 gram of material rich in phosphorus, are heated
in a conical flask with nitric acid of D = 1-2 (about 12 c.c. of acid per gram
of metal) and the solution treated with 1-2 c.c. of hydrofluoric acid -1 to
eliminate the silica and then heated for a few minutes longer.2 To oxidise
the phosphorus completely, 5-10 c.c. of 2% permanganate solution are
added with shaking and the heating continued for 2-3 minutes, 3-4 c.c.
of 25% potassium oxalate being then added and the liquid heated until
the separated manganese dioxide completely dissolves. Ammonia is next
added, drop by drop, to the clear solution until the first flocks of ferric
hydroxide separate, these being dissolved by a few drops of nitric acid.
The liquid is then transferred to a beaker and evaporated to a syrup, allowed
to cool to 50° and treated with 50-100 c.c. of ammonium molybdate solution
previously heated to 50°, and, after some time, 15-20 grams of solid ammo-
nium nitrate, which is dissolved by shaking. - After the beaker has been
left for about an hour at 45-50°, the phosphomolybdate precipitate is
collected on a tared Gooch crucible containing asbestos which has been
previously ignited and washed with nitric acid and then with water. The
precipitate is washed with water acidified with i% of nitric acid of D = 1-2
until the liquid passing through is free from iron and is then heated in an
1 The hydrofluoric acid is poured in from a platinum dish or crucible in such a -way
that it does not come into contact with the walls of the flask.
2 If the sample contains much graphitic carbon the solution, after treatment with
hydrofluoric acid, is made up to a definite volume and filtered through a dry filter, an
aliquot part being taken, treated with permanganate, and so on.01 3'60 ;